Maybe you understood, but you just didn't express it correctly:

Compiled object files are generated by a High-Level Language Compiler - it is most unlikely that you could use the compiled object files from Keil with SDCC.

However, there shouldn't be any reason why you can't use Keil's A51 Assembler-generated object files with SDCC.

Your assembler source for use with SDCC will, of course, still have to play by the SDCC rules.

You need to clearly understand the difference between an Assembler and a Compiler here - the terms are not equivalent, and are not interchangeable!